It has long been known that the proper amount of refrigerant charge in compression cycle refrigeration-systems is essential to system reliability and efficiency. Numerous schemes for providing the proper charge of refrigerant to refrigeration systems have been disclosed such as in U.S. Pat. Nos. 3,400,552; 3,791,165; and 3,875,755. Overcharge often results in compressor slugging with attendant valve failure. Undercharge may result in reducing cooling capacity and for those systems using refrigerant-cooled compressor motors, may result in motor overheating and burnout. Establishing the proper charge is most critical in refrigeration systems using a capillary tube type throttling means.
It has been the practice of manufacturers to design refrigeration equipment so that when properly charged, refrigerant will return to the compressor with a predetermined degree of superheat, such as 15.degree. F., where the refrigeration equipment is operated under certain standard conditions.
These standard conditions are often selected as 80.degree. F. dry bulb indoor temperature, 67.degree. F. wet bulb indoor temperature and 95.degree. F. dry bulb outdoor temperature.
When charging a refrigeration apparatus in the field it is not likely that these standard conditions will exist. Further, when refrigerant is added, transient pressure conditions exist which make it difficult to determine superheat by directly measuring suction line pressure.